1. Field of the Invention
This invention relates to the field of drive current reduction circuits, particularly circuits used to limit the drive current supplied to a low dropout voltage regulator's pass transistor.
2. Description of the Related Art
A basic "low dropout" voltage regulator (LDO) is shown in FIG. 1. Voltage regulators of this type provide a desired regulated output voltage as long as the input voltage is higher than the desired output voltage by at least the "dropout" voltage, typically about 100 mv. An input voltage V.sub.in is connected to the emitter 10 of a "pass transistor" 12, typically a pnp bipolar transistor, and an output voltage V.sub.out is taken at the transistor's collector 14 and drives a load R.sub.load. The output voltage is regulated by controlling pass transistor 12 via its control input 16. Regulation is accomplished with a feedback loop: the output voltage is fed back to the non-inverting input 18 of a loop amplifier 20, usually via a voltage divider 22. A voltage reference V.sub.ref is connected to the inverting input 24 of the amplifier. The amplifier's output is connected to the pass transistor's control input 16.
In operation, amplifier 20 drives the pass transistor as necessary to make the voltage at its inputs 18 and 24 equal, thereby holding the output voltage at a constant value proportional to the reference voltage. Unequal voltages at inputs 18 and 24 indicate that the output voltage is going out of regulation, and the amplifier adjusts the drive to restore the output to its desired value.
Two possible causes for the output voltage going out of regulation are: 1)a load current which exceeds the regulator's capabilities is demanded by load R.sub.load, and 2)a falling input voltage causes the voltage across the pass transistor to fall below the dropout voltage--a common occurrence for a battery-powered regulator as its battery nears discharge. In either case, the amplifier tries to restore the output by calling for more drive. When a falling input voltage is the cause and the regulator is lightly loaded, the increase in drive current may greatly exceed the actual load current. This is undesirable in battery applications: for example, suddenly increasing the load on the battery as it nears discharge can shorten its life.
One technique for reducing excessive drive current is to connect a diode across the base-collector junction of pass transistor 16. The diode, with its anode connected to the transistor's base, becomes forward-biased when V.sub.in and V.sub.out get sufficiently close together, reducing the drive to pass transistor 12 below that provided by amplifier 20. One disadvantage of this solution is that drive current is reduced regardless of the cause of the low differential voltage across transistor 12. Under some conditions, when V.sub.in is falling and the output is lightly loaded, for example, it may be desirable to allow the differential to fall lower than the point at which the diode begins conducting without affecting the transistor's drive. Another disadvantage is that the regulator's ground current will increase when amplifier 20 calls for more drive and the diode is conducting. Since ground current is power consumed from the source of V.sub.in such as a battery, it is desirable that it be kept as low as possible.
Another approach that has been taken is shown in FIG. 1. A transistor 26, shown here as a pnp, is connected across pass transistor 12: the base of transistor 26 is connected to the base of transistor 12, its emitter is connected to V.sub.oUt, and its collector is connected to amplifier 20 and arranged to reduce the drive to transistor 12 when transistor 26 is conducting. As with the diode, the transistor begins conducting when V.sub.in and V.sub.out get sufficiently close. However, this solution can also become active at a higher-than-desired differential voltage, causing the drive to be reduced while the regulator is still capable of maintaining the output voltage.